The present invention is directed to preforms for blow molding plastic containers, and to methods of making such preforms.
In the manufacture of plastic containers, it is conventional to injection mold or compression mold a container preform having a body and a finish with one or more external threads. The finish typically is molded to its final geometry, while the body of the preform is subsequently blow molded to the desired geometry of the container body. The preform may be of monolayer construction, or may be of multilayer construction in which one or more intermediate layers in the preform body may or may not extend into the finish area of the preform. U.S. Pat. Nos. 4,609,516, 4,710,118 and 4,954,376 illustrate injection molding of multilayer container preforms.
Molding the finish portion of the container as part of the preform presents a number of problems. For example, when the preforms are formed by injection molding, the plastic material typically is injected into a mold cavity at the closed end of the preform body, so that the material must flow along the sides of the preform cavity into the area in which the finish is molded. The finish typically requires more accurate and stable dimensioning than the body of the preform, which may limit the cycle time of the molding process. Furthermore, the finish portion of the preform is of the same material as at least the outer layers of the preform body, which limits the ability to obtain the most desirable characteristics at the finish. When the preform is of polyester construction, such as polyethylene terephthalate (PET), the finish portion of the preform can be wholly or partially crystallized to improve the operating characteristics of the finish area, particularly in hot-fill container applications. However, the requirement that the finish be of the same material as at least the outer layers of the preform body still limits the design capabilities of preform manufacture.
A preform assembly for blow molding a container in accordance with a first aspect of the present invention includes a molded plastic preform having a body and a neck with an S external surface. A plastic finish ring is molded separately from the preform and has an internal surface. The finish ring is secured over the preform neck with the surfaces in facing engagement. One or both of the external and internal surfaces are defined by a plurality of spaced lands and a plurality of open spaces between the lands. The open spaces between the lands reduce heat transfer between the preform neck and the finish ring. In exemplary preferred embodiments of the invention, the lands include either spaced axial ribs or spaced circumferential ribs, and the open spaces include either spaced axial channels or spaced circumferential channels between the ribs.